The present invention provides new coordinative catalysts for polymerization of alkylene oxides.
Polyalkylene oxides which are obtainable by polymerization of alkylene oxides, e.g. ethylene oxide, propylene oxide or 1,2-butylene oxide, are used commercially in a number of applications such as non-ionic surfactants, lubricants, braking fluids or hydraulic fluids. If the alkylene oxides are polymerized in the presence of starter compounds with active hydrogen atoms, polyether polyols are obtained, and these are used widely for producing polyurethane materials such as paints, sealants, elastomers or foams.
Polymerization of alkylene oxides may be catalyzed by a basic, acid or coordinative mechanism. The basic catalysts employed in industry for polymerization of alkylene oxides are chiefly caustic alkalis (e.g. KOH). The disadvantages of polymerization catalyzed by caustic alkalis are the long reaction times and the very expensive processing of the product to separate the basic catalyst. Another problem is the transposition of alkylene oxides such as propylene oxide with a basic catalyst to form allyl or propenyl alcohols, which takes place as a side reaction, the alcohols giving monofunctional, unsaturated polyethers with a double bond on the end, so-called monols, in the production of polyether polyol. Since the proportion of monool increases greatly as the molar weight of polyether rises, the equivalent molar weight (numerical mean molar weight/functionality) is limited to about 2000 g/mol in polyether polyol production by means of KOH catalysis.
As well as basic catalysis, acid catalysis, particularly with Lewis acids e.g. boron trifluoride, has long been known for polymerization of alkylene oxides. The drawback of acid catalysis is that it further encourages side reactions (e.g. formation of volatile, low molecular weight cyclic ethers such as dioxans or dioxolans), so generally speaking only products with numerical mean molar weights up to about 1000 g/mol can be obtained, and molar weight distribution in polyalkylene oxides is wider than in products made by basic catalysis.
Coordinative catalysts for polymerization of alkylene oxides have also been known for quite a long time. The first catalysts described for coordinative polymerization of alkylene oxides were iron chloride, diethyl zinc and various trialkylaluminium compounds with additives and co-catalysts. The disadvantages of these first coordinative catalysts for alkylene oxide polymerization were their relatively low activity and the difficulty of separating them from the product. As chain exchange between the growing polyalkylene oxide chain and an added starter compound is very slow with these catalyst systems, the molar weight of the polymers and often also the end group functionality cannot be effectively controlled, which makes the products unsuitable for polyurethane applications. In addition, these catalyst systems produce parts of stereoregular polyethers in some cases.
Porphyrin complexes of aluminum, zinc and manganese also catalyze polymerization of alkylene oxides by a coordinative mechanism (see e.g. EP-A 195 951, EP-A 510 602, U.S. Pat. No. 5,328,970). The polyalkylene oxides obtained are atactic and have a narrow molar weight distribution. A general problem with the use of these metalloporphyrin complexes, however, is completely separating the strongly colored catalyst systems from the product. As the catalysts are also very expensive to prepare, metalloporphyrins are unsuitable for producing polyalkylene oxides on an industrial scale for economic reasons.
Double metal cyanide (DMC) catalysts based on zinc hexacyanocobaltate, when modified by suitable organic complex ligands, are very effective catalysts for coordinative alkylene oxide polymerization (see e.g. U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849, 5,158,922, 5,470,813). Atactic polyalkylene oxides with very narrow molar weight distributions and an extremely low content of unsaturated by-products are obtained by employing highly active DMC catalysts. DMC catalysts however have the disadvantage of not allowing controlled polymerization of ethylene oxide and hence also not allowing production of polyethylene oxides or polyalkylene oxide block copolymers with ethylene oxide blocks. In addition, the induction period characteristic of DMC catalysis at the beginning of polymerization frequently causes processing problems, particularly in large-scale industrial processes.
Improved catalyst systems for coordinative polymerization of alkylene oxides leading to atactic polyalkylene oxides with a narrow molar weight distribution and low proportions of unsaturated by-products are therefore of great interest.